A Prefetching Scheme Exploiting both Data Layout and Access History on Disk

Prefetching is an important technique for improving effective hard disk performance. A prefetcher seeks to accurately predict which data will be requested and load it ahead of the arrival of the corresponding requests. Current disk prefetch policies in major operating systems track access patterns at the level of file abstraction. While this is useful for exploiting application-level access patterns, for two reasons file-level prefetching cannot realize the full performance improvements achievable by prefetching. First, certain prefetch opportunities can only be detected by knowing the data layout on disk, such as the contiguous layout of file metadata or data from multiple files. Second, nonsequential access of disk data (requiring disk head movement) is much slower than sequential access, and the performance penalty for mis-prefetching a randomly located block, relative to that of a sequential block, is correspondingly greater.